The present invention relates to magnetic field sensors used in sensing gear rotation in general, and to Hall effect sensors mounted to detect ferrous gear teeth in particular.
In order to control a motor using feedback, it is necessary to employ sensors that detect the operation of the motor. If the motor is used to drive a ferrous gear, a simple and reliable sensor that can detect rotation of the ferrous gear can be constructed based on a Hall effect sensor. Sophisticated sensor modules that incorporate a magnet and a ferrous pole piece and one or two Hall effect sensors are available from a number of manufacturers. When the sensor module is placed so that the Hall effect sensors are within about zero to 2.5 mm from the peripheral edge of a ferrous gear, passage of the gear tooth past the Hall effect sensors creates an increase in magnetic field strength which is detected by the Hall effect sensors. By using a sensor module that incorporates two spaced Hall effect sensors it is possible to detect both speed and direction of motion of the gear when the Hall effect sensors are arranged to be spaced in the circumferential direction of motion of the gear. However, greater sensitivity and simplicity in processing the sensor's output can be accomplished by using two sensor modules that are spaced apart, and also arranged such the spacing is a multiple of half the width of the gear tooth, so as to produce a quadrature output which allows determination of the direction of motion of the gear.
In order to have better resolution for smaller toothed gears it is advantageous to control the spacing of the two Hall effect sensors within a single sensor module. Without the considerable extra expense of producing a unique sensor for a particular application, it is possible to adjust the effective spacing between the Hall effect sensors within a single sensor device by rotating the sensor module and thus the included Hall effect sensors in the sensor plane so that, relative to a line at the leading edge of an advancing gear tooth, the distance between the Hall effect sensors is foreshortened. If two Hall effect sensors that have been rotated to control the effective spacing between Hall effect sensors on each sensor module are to be used, a problem arises in mounting the two sensor modules in the proper rotation and angular position relative to each other and to the motor gear.